1. Field of the Invention
The present invention relates to a noise reduction assembly, particularly for use with an exhaust system of an engine.
2. Description of the Related Art
Automobile engines are generally constructed with an exhaust silencer or muffler unit connected with the exhaust gas passageway. Various muffler constructions have been suggested to reduce the exhaust noises associated with internal combustion engines without affecting the performance of the engine.
Another noise associated with the exhaust system of an engine is referred to as “tick and ping” noise. This noise is produced by thermal growth movement of two mating components, such as an exhaust pipe assembly, or exhaust silencer tubes, or a traditional round or bottle resonator. Usually, the components include a tube placed inside another tube or a tube having an end crimped or pinched over another tube. When the temperature of one or both tubes increases, due to various causes such as hot exhaust gas moving through one or both tubes, the tubes expand radially and lengthwise. The inner tube becomes in a tight contact with the outer tube, while the inner tube slides against the outer tube. When the temperature drops, the tubes contract, causing a movement that produces “tick and ping” noise.
It should also be observed that, tick and ping noise may be generated as the exhaust system temperature increases. However, the exhaust system temperature increases while the automobile is in operation. Accordingly, background noise associated with the operation of the automobile, road noise, or perhaps an operating radio, serve to obscure “tick and ping” noise as the exhaust system temperature increases.
In contrast, in the absence of background noise, the same noise generated upon cooling of the exhaust system may annoy consumers.
Traditional exhaust noise mufflers are effective over a part of the range of frequencies generated by internal combustion engines. A solution to the engine noise not removed by a traditional muffler is to connect a resonator, such as a bottle resonator, in series with the muffler where the bottle resonator is tuned to remove noise frequencies not removed by the muffler. However, the resonator may be the source of “tick and ping” noise, based on the traditional construction of the resonator, which requires a tube within a tube assembly, as described above.
Another example of ways to attenuate sound in the muffler is to use an absorptive fibrous material packed into sound absorption chambers in the muffler. For example, U.S. Pat. No. 4,396,090 shows a muffler in which each absorption chamber is completely filled with mineral wool. Although sound attenuation of certain higher frequency ranges is achieved using such chamber-filling materials, the manufacturing cost of such a design is high because of the large quantity of fibrous material needed to fill one or more of the muffler sound absorption chambers. In U.S. Pat. No. 4,930,597, a tubular sock made of a fibrous material placed around a louvered exhaust tube, provides a high-frequency noise attenuation filter.
The above known solutions to the engine noise do not address the problem of “tick and ping” noise generated from thermal expansion and contraction of exhaust system components. Therefore, there is a need to reduce the unpleasant “tick and ping” noise, coupled with reducing the engine noise.